In the printing pre-press industry, it is often necessary to retain a plate or sheet of media on a surface so that it can be imaged. Typically, an imaging source is scanned relative to the surface of the media by either moving the imaging source or the media or a combination thereof. For example, many computer-to-plate or computer-to-press systems image a lithographic printing plate that is held onto the outside surface of a rotating drum. Systems are also available for imaging a plate held on the internal surface of a cylinder or on a flat platen.
Commonly assigned U.S. Pat. No. 6,130,702 to Ganton shows a combination of a mechanical reference edge and clamp for retaining the leading edge of a plate and magnetic clamps for retaining the trailing edge of the plate. The leading edge clamp is usually fixed in location while the magnetic clamp can be placed in a variety of locations to suit a range of plate sizes. The drum is made of a ferromagnetic material such as cast iron or has ferromagnetic inserts.
There remains a need for better magnetic clamps for holding media to imaging beds. There is a particular need for such clamps that provide increased holding forces and can accommodate media of different thicknesses.